The present invention relates to a photovoltaic cell which is excellent in photovoltaic transduction efficiency (photovoltaic transfer efficiency).
More particularly, the present invention relates to a photovoltaic cell which realizes high electron moving velocity, ensures prolonged stability of electrolyte layer and exhibits high photovoltaic transduction efficiency. Still more particularly, the present invention relates to a thin photovoltaic cell for solar cell, or thin flexible photovoltaic cell for solar cell, exhibiting high energy transduction efficiency.
Photovoltaic transducers are a material from which light energy is continuously taken out as electric energy and a material which converts light energy to electric energy by the utilization of an electrochemical reaction between electrodes. When a photovoltaic transducer material is irradiated with light, electrons are generated from one electrode. The electrons move to a counter electrode, and the electrons having reached the counter electrode return by migrating as ions through an electrolyte to the one electrode. This energy conversion is continuously carried out, so that it is utilized in, for example, a solar cell.
The common solar cell is produced by first forming an electrode on a support such as a glass plate coated with a transparent conductive film, subsequently forming a semiconductor film having a photosensitizer adsorbed thereon on a surface of the electrode, thereafter providing a counter electrode comprising a support such as a glass plate coated with another transparent conductive film, sealing an electrolyte between the counter electrode and the semiconductor film, and finally sealing the side faces with a resin or the like.
When the above semiconductor film is irradiated with sunlight, the photosensitizer adsorbed on the semiconductor absorbs visible-region rays to thereby excite itself. Electrons generated by this excitation move to the semiconductor, next to the transparent conductive glass electrode, and further to the counter electrode across a lead connecting the two electrodes to each other. The electrons having reached the counter electrode reduce the oxidation-reduction system in the electrolyte. On the other hand, the photosensitizer having caused electrons to move to the semiconductor is in oxidized form. This oxidized form is reduced by the oxidation-reduction system of the electrolyte to thereby return to the original form. In this manner, electrons continuously flow. Therefore, functioning as the solar cell can be realized.
The electrolyte to be sealed between the electrodes is dissolved in a solvent, selected according to the type of the electrolyte, to thereby obtain an electrolytic solution. The electrolytic solution is sealed in a cavity created by sealing the sides of the photovoltaic cell with, for example, a resin.
The above solvent can be selected from among, for example, water, alcohols, oligoesters, carbonates such as propione carbonate, phosphoric acid esters, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, N-vinylpyrrolidone, sulfur compounds such as sulfolane 66, ethylene carbonate and acetonitrile.
However, when the electrolyte is used in the form of such an electrolytic solution, the solar cell may suffer from, during a long-term service, alteration of solvent molecules, decomposition of solvent molecules, vaporization of low-boiling-point solvent, leakage of electrolytic solution (solvent and/or electrolyte) from sealed parts, etc. with the result that the performance, such as photovoltaic transduction efficiency, thereof is deteriorated. That is, the use of the electrolyte in the form of an electrolytic solution has a drawback in that the long-term stability is poor.
Moreover, depending on the type of electrolyte used in the electrolytic solution, hygroscopicity is exhibited to thereby absorb water, and it may occur that the water causes the electrolyte and the photosensitizer to decompose to result in performance deterioration.
The photovoltaic transduction efficiency of the current solar cell is not always satisfactory, and there are limitations in the application thereof. Therefore, there is a demand for further enhancement of light utilization ratio.
It is an object of the present invention to provide a photovoltaic cell which is excellent in long-term stability, ensures high light utilization ratio and exhibits high photovoltaic transduction efficiency.
The first photovoltaic cell of the present invention comprises:
a first substrate (base) having on its surface an electrode layer (1), the electrode layer (1) having on its surface a semiconductor film (2) on which a photosensitizer is adsorbed, and
a second substrate having on its surface an electrode layer (3),
the first substrate and the second substrate arranged so that the electrode layer (1) overlaid with the semiconductor film (2) and the electrode layer (3) are opposite to each other with an electrolyte layer (4) interposed therebetween,
wherein the electrolyte layer (4) comprises an electrolyte and a liquid crystal, and
wherein at least one of the electrode-layer-having substrates is transparent.
The first photovoltaic cell of the present invention contains a liquid crystal in the electrolyte layer thereof, so that, even if the angle of light incidence is large, the quantity of light received is not much decreased by virtue of the light scattering depending on the existence of the liquid crystal with the result that light energy can be stably transduced to electrical energy and taken out.
The second photovoltaic cell of the present invention comprises:
a first substrate having on its surface an electrode layer (21), the electrode layer (21) having on its surface a semiconductor film (22) on which a photosensitizer is adsorbed, and a second substrate having on its surface an electrode layer (23),
the first substrate and the second substrate arranged so that the electrode layer (21) overlaid with the semiconductor film (22) and the electrode layer (23) are opposite to each other with an electrolyte sealed between the semiconductor film (22) and the electrode layer (23),
wherein spacer particles are interposed between the semiconductor film (22) and the electrode layer (23), and wherein at least one of the electrode-layer-having substrates is transparent.
In the second photovoltaic cell of the present invention, spacer particles are interposed between the semiconductor film and the electrode layer opposite thereto, so that not only can the inter-electrode gap be decreased and uniformized but also the energy loss of charges (electrons) moving through the electrolyte layer can be reduced. Moreover, the whole cell can exhibit uniform optical and electrical characteristics. Therefore, the photovoltaic transduction efficiency can be enhanced, and the amount of electrolyte can be reduced.
The third photovoltaic cell of the present invention comprises:
a first substrate having on its surface an electrode layer (31), the electrode layer (31) having on its surface a semiconductor film (32) on which a photosensitizer is adsorbed, and
a second substrate having on its surface an electrode layer (33),
the first substrate and the second substrate arranged so that the electrode layer (31) overlaid with the semiconductor film (32) and the electrode layer (33) are opposite to each other with an electrolyte sealed between the semiconductor film (32) and the electrode layer (33),
wherein spacer particles are sunk in the semiconductor film (32) in such a manner that at least portions of the spacer particles are exposed from the semiconductor film (32) so as to contact the electrode layer (33).
In the third photovoltaic cell of the present invention, the semiconductor film in which spacer particles are partly sunk is disposed opposite to the electrode layer with spacer particles interposed therebetween, so that not only can the inter-electrode gap be decreased and uniformized but also the energy loss of charges (electrons) moving through the electrolyte layer can be reduced. Moreover, the whole cell can exhibit uniform optical and electrical characteristics to thereby enable improving apparent curve factor (FF) and configuration factor. Therefore, high photovoltaic transduction efficiency can be exhibited. Furthermore, by virtue of the interposition of spacer particles, contacting of the semiconductor film with the electrode layer opposite thereto can be avoided even when pressure is applied to the cell. Still further, the electrolyte layer can be made uniform and extremely thin, so that the light absorption by the electrolyte can be reduced. Still further, electromotive force is also effectively produced by incident light from the side of electrode opposite to the semiconductor film. Therefore, the photovoltaic cell can appropriately be used as a thin, or thin flexible, film solar cell.
In the above second and third photovoltaic cells, it is preferred that the spacer particles be spherical particles having an average diameter (D) of 1 to 100 xcexcm.
Further, it is preferred that a surface of the semiconductor film (22), (32) that is brought into contact with the spacer particles have a roughness (RS) of 5 xcexcm or less, and also a surface of the electrode layer (23), (33) that is brought into contact with the spacer particles have a roughness (RE) of 5 xcexcm or less, and that the ratio of surface roughness (RS) to average diameter of spacer particles (D), RS/D, and the ratio of surface roughness (RE) to average diameter of spacer particles (D), RE/D, be both 0.2 or less.
The above semiconductor film (2), (22), (32) is preferably a metal oxide semiconductor film.
The spacer particles are preferably particles having a hydrophobic (water repellent) surface, and a dispersion medium thereof is preferably a solvent which is miscible with water. The component for semiconductor film formation preferably comprises particles of crystalline titanium oxide.
The fourth photovoltaic cell of the present invention comprises:
a first substrate having on its surface an electrode layer (41), the electrode layer (41) having on its surface a metal oxide semiconductor film (42) on which a photosensitizer is adsorbed, and
a second substrate having on its surface an electrode layer (43),
the first substrate and the second substrate arranged so that the metal oxide semiconductor film (42) and the electrode layer (43) are opposite to each other with an electrolyte layer interposed therebetween,
wherein:
(i) the metal oxide semiconductor film (42) comprises metal oxide particles having an average diameter of 5 to 600 nm,
(ii) the metal oxide particles each have a core/shell structure comprising a core particle part and, disposed on a surface thereof, a shell part,
(iii) the core particle parts have an average diameter of 2 to 500 nm, and the shell parts have a thickness ranging from 1 to 150 nm, and
(iv) the metal oxide constituting the core particle parts and the metal oxide constituting the shell parts have intrinsic volume resistivity values (Ec) and (Es), respectively, which satisfy the relationship:
Ec less than Es.
In the fourth photovoltaic cell, it is preferred that the metal oxide constituting the shell parts be crystalline titanium oxide. The crystalline titanium oxide is preferably one obtained by heating/aging of peroxotitanic acid. Further, it is preferred that the metal oxide semiconductor film comprise metal oxide particles and a titanium oxide binder.
The coating liquid for forming a semiconductor film for use in a photovoltaic cell according to the present invention comprises a component for semiconductor film formation and spacer particles both dispersed in a dispersion medium. Peroxotitanic acid is preferably contained as the binder component. The average diameter of spacer particles is preferably in the range of 1 to 100 xcexcm.